Floppy disk devices are used in a large quantity for information recording of e.g. computers or wordprocessors. Usual floppy disk devices had a problem that since positioning of the head is conducted by an open loop control using a step motor (control without feedback), positioning precision is poor, and it is impossible to increase the track density.
In recent years, it has been proposed to provide grooves on a magnetic recording medium and monitoring the position of the head by reading the groove position by an optical sensor provided integrally with the head. According to this method, positioning of the head is conducted by a closed loop control (feedback control), whereby positioning precision can be improved, and it is possible to realize a track density higher by one figure than the conventional devices.
One example of a floppy disk utilizing this principle is shown in FIGS. 11 and 12. Reference numeral 19 indicates a floppy disk, and FIG. 12 is an enlarged view of a part of the magnetic recording medium 4 of FIG. 11. FIG. 14 is a cross-sectional view vertical to the medium plane, illustrating a servo-signal reading system of an optical track servo-mechanism. On the surface of the magnetic recording medium 4, numerous pits 5 are formed in accordance with a track pitch. Through an aperture formed in the center of a head 6, a light from a light-emitting element 8 is projected on the surface of the magnetic recording medium 4. Tracking is carried out by reading a reflected light thereof by a light receptor 10 via an optical system 9.
An optical detector and a tracking error detecting circuit used for this purpose are shown in FIGS. 13 and 15, respectively. In the following description, P represents a track pitch.
The light receptor 10 is composed of four unit elements arranged in a square lattice pattern and upon receiving the above-mentioned reflected light, outputs four signals A to D. When the head is located at the track position (radius R), signal B is deducted from signal A by a differential amplifier 11 to obtain a signal proportional to cos(2.pi.R/P). Likewise, signal D is deducted from signal C by a differential amplifier 12 to obtain a signal proportional to sin(2.pi.R/P). On the other hand, binary codes for target value T are input to address terminals of ROM 13 and 14 in which a sin table and a cos table are written, to prepare binary codes representing sin(2.pi.T/P) and cos(2.pi.T/P). These binary codes are converted to analogue signals by multiplication type DA converters 15 and 16, and at the same time, their multiplication with the above-mentioned signals sin(2.pi.R/P) and cos(2.pi.R/P) obtained from the optical detector is conducted as shown by the following formulas, and then the difference is taken by a differential amplifier 17 whereby an error signal is obtained by an operation as shown by the following formulas. ##EQU1##
By feeding back this error signal to the tracking device, it is possible to carry out tracking with high precision with an error being almost 0. Such a tracking servo-device had problems such that two sets of detecting devices are required, the sizes of the devices are large, and costs required for their production are high.
A magnetic recording medium to be used for such a method is required to have grooves (pits) preliminarily formed on its surface. As a method for forming such grooves, a method of pressing to the medium a die having convexes formed to correspond to the grooves, to transfer the shapes of the die to the medium (stamping processing) and a method of irradiating a laser beam to decompose and remove part of the magnetic layer (laser processing) are known.
However, these methods have had problems such that in each case, the processing apparatus is extensive, a dust is generated during the processing and a cleaning step is therefore required, and the production costs are high. Further, the completed medium has grooves on its surface and thus has had a problem that such areas are not suitable for magnetic recording, whereby the recording capacity is reduced. Furthermore, there has been a problem that the difference in the optical property due to the presence or absence of such grooves is small, the detecting sensitivity is low, or it is susceptible to the influence of noises.